way or that way, change either into yellow and red, as in the posture
_de_, or into blue and violet, as in the posture [Greek: de]. And if the
Light before it fall upon the Paper be twice refracted the same way by
two parallel Prisms, these Colours will become the more conspicuous.
Here all the middle parts of the broad beam of white Light which fell
upon the Paper, did without any Confine of Shadow to modify it, become
colour'd all over with one uniform Colour, the Colour being always the
same in the middle of the Paper as at the edges, and this Colour changed
according to the various Obliquity of the reflecting Paper, without any
change in the Refractions or Shadow, or in the Light which fell upon the
Paper. And therefore these Colours are to be derived from some other
Cause than the new Modifications of Light by Refractions and Shadows.
If it be asked, what then is their Cause? I answer, That the Paper in
the posture _de_, being more oblique to the more refrangible Rays than
to the less refrangible ones, is more strongly illuminated by the latter
than by the former, and therefore the less refrangible Rays are
predominant in the reflected Light. And where-ever they are predominant
in any Light, they tinge it with red or yellow, as may in some measure
appear by the first Proposition of the first Part of this Book, and will
more fully appear hereafter. And the contrary happens in the posture of
the Paper [Greek: de], the more refrangible Rays being then predominant
which always tinge Light with blues and violets.
_Exper._ 4. The Colours of Bubbles with which Children play are various,
and change their Situation variously, without any respect to any Confine
or Shadow. If such a Bubble be cover'd with a concave Glass, to keep it
from being agitated by any Wind or Motion of the Air, the Colours will
slowly and regularly change their situation, even whilst the Eye and the
Bubble, and all Bodies which emit any Light, or cast any Shadow, remain
unmoved. And therefore their Colours arise from some regular Cause which
depends not on any Confine of Shadow. What this Cause is will be shewed
in the next Book.
To these Experiments may be added the tenth Experiment of the first Part
of this first Book, where the Sun's Light in a dark Room being
trajected through the parallel Superficies of two Prisms tied together
in the form of a Parallelopipede, became totally of one uniform yellow
or red Colour, at its emerging out of the Prisms. Here, in the
production of these Colours, the Confine of Shadow can have nothing to
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